1. Field of the Invention
The present invention relates to a film formation apparatus for a semiconductor process for forming a film on a target substrate, such as a semiconductor wafer, and also to a method for using the apparatus. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In the process of manufacturing semiconductor devices, a process is performed for forming a silicon nitride film on the surface of a target substrate, such as a semiconductor wafer (which will be also simply referred to as “wafer” hereinafter). Silicon nitride films are widely used, because they have better insulation properties as compared to silicon oxide films, and they can sufficiently serve as etching stopper films or inter-level insulating films. For example, a silicon nitride film is formed by CVD (Chemical Vapor Deposition) using a reaction between a silane family gas, such as dichlorosilane (SiH2Cl2: DCS), and ammonia (NH3) gas.
In a heat processing apparatus, as a process for forming a silicon nitride film is repeatedly performed, by-product films are deposited and gradually accumulated on the inner surface of the process container (reaction chamber) and wafer holder. The by-product films thus formed are partly peeled off and generate particles when the interior of the process container is heated for a subsequent heat process after the cumulative film thickness reaches a predetermined thickness. The particles may drop on semiconductor wafers, and reduce the yield of semiconductor devices to be fabricated. Further, the by-product films may emit gases, which cause a reaction and thereby deposit unexpected components on wafers.
U.S. Pat. No. 6,573,178 B1 discloses a technique for dealing with the problem described above. According to this technique, when the cumulative film thickness of a film formation process reaches a predetermined thickness, a purge process is performed after processed wafers W are unloaded from the process container and before subsequent unprocessed wafers W are loaded into the process container. For example, where the target film thickness of a film formation process performed once is larger than the predetermined thickness, the purge process accompanies the film formation process every time. According to this purge process, while an inactive gas, such as nitrogen gas, is supplied into the process container, the interior of the process container is cooled from the film formation temperature. This is intended to positively apply a thermal stress to by-product films inside the process container to crack the films and thereby positively peel off and remove surface portions of the films. The film portions removed by this purge process are surface portions of the films deposited inside the process container in a sate where they can easily become particles. However, where these portions are removed, generation of gases and/or particles is suppressed in the film formation process performed subsequently to the purge process.
In recent years, owing to the demands of increased miniaturization and integration of semiconductor integrated circuits, it is required to alleviate the thermal history of semiconductor devices in manufacturing steps, thereby improving the characteristics of the devices. For vertical processing apparatuses, it is also required to improve semiconductor processing methods in accordance with the demands described above.
U.S. Pat. No. 7,094,708 B2 discloses a structure of a vertical processing apparatus for performing a molecular layer deposition method, which utilizes plasma assistance to further decrease the process temperature. According to this apparatus, for example, where dichlorosilane (DCS) and NH3 are used as a silane family gas and a nitriding gas, respectively, to form a silicon nitride film (SiN), the process is performed, as follows. Specifically, DCS and NH3 gas are alternately and intermittently supplied into a process container with purge periods interposed therebetween. When NH3 gas is supplied, an RF (radio frequency) is applied to generate plasma so as to promote a nitridation reaction. More specifically, when DCS is supplied into the process container, a layer with a thickness of one molecule or more of DCS is adsorbed onto the surface of wafers. The superfluous DCS is removed during the purge period. Then, NH3 is supplied and plasma is generated, thereby performing low temperature nitridation to form a silicon nitride film. These sequential steps are repeated to complete a film having a predetermined thickness.